1. Field of the Invention
The present invention relates to a method for monitoring a width of a fine pattern with a high degree of accuracy and further relates to an exposure method, an etching method, a manufacturing method for a semiconductor device and an exposure processing unit, all of which use the monitoring method.
2. Description of the Related Art
A semiconductor device performance is mostly controlled by a dimensional precision of a fine pattern. In microfabrication, by using a resist pattern formed by photolithography as a mask, an underlying film such as an insulating film, a conductive film or the like is subjected to a dry etching processing. In manufacturing of a semiconductor device, improvement of dimensional precision in processing in a semiconductor substrate surface is a significant issue. It is thus required to control, with a higher degree of accuracy, parameters for improving the dimensional precision in the processing of the semiconductor substrate by photolithography and dry etching.
In a photolithography process, a semiconductor device pattern is transferred to a resist film serving as a photosensitive agent, coated on a semiconductor substrate, by using an aligner, for example, a reduction projection aligner (stepper). To be more specific, light emitted from a light source is transmitted through a reticle, on which a semiconductor device pattern to be transferred is delineated, and then is reduced by an optical system. In this way, the light is projected onto the semiconductor substrate, so as to form a resist pattern. In the pattern formation by using the stepper, a resolution of the stepper is proportional to an exposure light wavelength λ and is inversely proportional to a numerical aperture NA, based on optical theory. Therefore, because of the demand for miniaturization of semiconductor device, process improvement associated therewith has been heretofore conducted, including shortening of the exposure light wavelength and achieving a higher NA of a projection lens. However, along with the recent demands for further miniaturization of the semiconductor device, it has become extremely difficult to ensure an exposure tolerance and a depth of focus. The depth of focus is proportional to the exposure light wavelength λ and is inversely proportional to the square of the numerical aperture NA. Thus, in order to make measurable improvements in the dimensional precision in processing by effectively utilizing a small exposure margin without causing a reduction in yield, i.e., the amount of production, an exposure dose and focus control with a higher degree of accuracy is required.
For example, when a semiconductor device image is projected repeatedly step by step on a fraction of a semiconductor substrate with the same exposure dose so as to delineate a number of semiconductor device patterns on the semiconductor substrate, the exposure condition varies in the semiconductor substrate surface because of the warpage of the semiconductor substrate, a film thickness distribution of the resist film on the semiconductor substrate surface, and an unevenness in the semiconductor substrate surface relating a post-exposure bake (PEB) and development or the like. Thus, a reduction in the production yield occurs.
Proposals have been made for an exposure monitoring method with attention on exposure control, in “A. Starikov, “Exposure monitor structure”, SPIE Vol. 1261 Integrated Circuit Metrology, Inspection, and Process Control 4 (1990)” and in Japanese Patent Laid-Open No. 2000-310850. The proposals have their own common characteristics in that, in a stepper, an image is transferred with a inclined exposure distribution by using a reticle having a pattern therein with a dimensional ratio (a duty ratio) of a transparent portion and an opaque portion continuously changed in one direction by a pitch that can not resolve images on the semiconductor substrate. By use of this method, a variation distribution of an effective optimum exposure dose in resist mask pattern formation can be provided.
Moreover, in Japanese Patent Laid-Open No. Hei 10(1998)-270320, an exposure method is proposed, in which, in order to reduce a variation of the dimensional precision in a pattern on a semiconductor substrate after a photolithography process, the semiconductor substrate is divided into several areas and an exposure dose is set for each of the areas. However, it seems that this method may not be an effective correction method because an approximation accuracy of a exposure dose is not so high.
A dry etching process is performed following the photolithography process. In the dry etching process, an underlying film on the semiconductor substrate is subjected to etching by using a resist pattern formed in the photolithography process as a selective etching mask and by bombarding ions generated in a gas plasma. The dimensional precision of the dry etching is not only influenced by etching conditions such as substrate temperature, plasma density, self-bias voltage and the like but also by the shape of the selective etching mask made of the resist pattern. Therefore, the respective influences of the dry etching and the photolithography cannot be separated from each other so as to be measured. Thus, direct monitoring of the exposure dose thereof may be impossible.
Moreover, in order to provide a microfabrication control with a high degree of accuracy, at the same time, a width monitoring technology of a fine pattern is also important. For the width monitoring of the fine pattern, a scanning electron microscope (SEM) has been heretofore used. However, measurement by the SEM has problems; (a) a sufficient measurement accuracy cannot be provided because pattern miniaturization has taken the SEM measurement approach to its limit; and (b) an electron beam irradiation is likely to damage the resist film, the underlying film or the semiconductor substrate.
As described above, in the microfabrication, in order to obtain a processing precision and evenness of a pattern dimension of the semiconductor device, it is important to control, with high accuracy, the exposure conditions of the photolithography, the etching conditions of the dry etching and the like. However, there is a problem in the method for measuring with easily high accuracy the fine pattern without damaging the semiconductor substrate. Moreover, it is difficult to separate the influences of the respective processes of the photolithography and the dry etching in the microfabrication for measurement with high accuracy. Thus, processing control of the semiconductor device cannot be effectively achieved.